Win32: make the lstat implementation pluggable

Emulating the POSIX lstat API on Windows via GetFileAttributes[Ex] is quite
slow. Windows operating system APIs seem to be much better at scanning the
status of entire directories than checking single files. A caching
implementation may improve performance by bulk-reading entire directories
or reusing data obtained via opendir / readdir.

Make the lstat implementation pluggable so that it can be switched at
runtime, e.g. based on a config option.

Signed-off-by: Karsten Blees <blees@dcon.de>

compat/mingw.c

@@ -574,6 +574,8 @@ static int do_stat_internal(int follow, const char *file_name, struct stat *buf)
 	return do_lstat(follow, alt_name, buf);
 }
 
+int (*lstat)(const char *file_name, struct stat *buf) = mingw_lstat;
+
 int mingw_lstat(const char *file_name, struct stat *buf)
 {
 	return do_stat_internal(0, file_name, buf);

compat/mingw.h

@@ -319,7 +319,7 @@ int mingw_fstat(int fd, struct stat *buf);
 #ifdef lstat
 #undef lstat
 #endif
-#define lstat mingw_lstat
+extern int (*lstat)(const char *file_name, struct stat *buf);
 
 #ifndef _stati64
 # define _stati64(x,y) mingw_stat(x,y)